This invention relates to a method and apparatus for producing a high density ceramic fuel pellet. In particular, the invention is directed to a method and apparatus for producing a high density ceramic pellet comprising ThO.sub.2 and UO.sub.2 having an outer annular shell comprising ThO.sub.2.
U.S. Pat. No. 3,325,419 to Hurley et al discloses nuclear fuel pellets comprising ThO.sub.2 and UO.sub.2 with a protective cladding layer of silica. The pellets are formed by a complicated procedure involving the formation of an aqueous sol of thoria and urania and a cladding solution of silica.
U.S. Pat. No. 3,755,513 to Stoll et al discloses a sintered ceramic oxide fuel material comprising UO.sub.2 or mixtures of UO.sub.2 with plutonium dioxide or thorium dioxide. The method disclosed by Stoll et al includes adding a compound which is decomposable at temperatures of up to 600.degree. C. to the UO.sub.2 powder or mixture of UO.sub.2 with plutonium dioxide or thorium dioxide and sintering at high temperatures.
Another procedure used to form a sintered ceramic oxide fuel comprises mixing in small quantities of solid organic additives with the ceramic oxide particles. The disadvantages of this procedure are the cost of the production of the fuel and the attendent necessary additional protective steps involved in using organic material.
A common problem associated with the fabrication of sintered high density pellets composed of ThO.sub.2 and UO.sub.2 is that some of the particles exposed on the pellet surface can be fractured during fabrication. Accordingly, if UO.sub.2 particles are present on the exposed pellet surface fracturing of these particles may occur. When these fractured particles are irradiated, they can be expected to extrude out from the ThO.sub.2 and approach, or contact, the inner surface of the fuel pellet cladding. This fuel-cladding contact is of great concern because it increases the potential for cladding rupture with concomitant fission product release. To reduce the potential for fuel-cladding contact, an annular shell of ThO.sub.2 has been proposed to act as a barrier to fuel extrusion. However, the formation of an annular contiguous bonded ThO.sub.2 shell is difficult because of the high potential for delamination. Prior attempts at forming this annular shell have not been successful. The present invention is directed to a method and apparatus for producing a sintered high densified ceramic having a bonded outer shell which prevents or substantially diminishes fuel-cladding contact.